Although biomass has long shown promise as a renewable source of fuel energy, there remains a need for more efficient means of transforming biomass into suitable biofuels. Cellulosic and lignocellulosic feedstocks and wastes, such as agricultural residues, wood, forestry wastes, sludge from paper manufacture, and municipal and industrial solid wastes, provide potential renewable feedstock for the production of valuable products such as fuels and other chemicals. The plant materials are also a significant source of fermentable sugar, such as glucose that can be transformed into biofuels. The sugars in plant material are composed of long chain of polymers comprising cellulose, hemicellulose, glucans and lignin. Thus it is necessary to break down these polymers into monomer sugars. The methods of converting biomass into fermentable sugars are known in the art and in general comprise of two main steps: a pretreatment step to loosen the plant structure, and a chemical or an enzymatic hydrolysis step to convert the polymeric chains of cellulose and hemicellulose into monomeric sugars, which can then be fermented to useful products.
The pretreatment step is a fine balancing act aimed at opening the fibre to enable enzyme accessibility while minimizing sugar loss and inhibitor generation to ensure high yields and substrate suitable for enzymatic hydrolysis and fermentation. Pretreatment methods are used to make the carbohydrate polymers of cellulosic and lignocellulosic materials more readily amenable to saccharification/hydrolysis enzymes.
The pretreated mixture is then subjected to enzymatic hydrolysis using enzyme such as hemicellulases and cellulases, which catalyze the hydrolysis of hemicellulose or cellulose to oligosaccharides and/or monosaccharides in the hydrolysate. The hydrolysate is further subjected to fermentation to produce biofuels. Saccharification enzymes used to produce fermentable sugars from pretreated biomass typically include one or more glycosidases, such as cellulose-hydrolyzing glycosidases, hemicellulose-hydrolyzing glycosidases, starch-hydrolyzing glycosidases, as well as peptidases, lipases, ligninases and/or feruloyl esterases. Saccharification enzymes and methods for biomass treatment are reviewed in Lynd, L. R. et al. (Microbiol. Mol. Biol. Rev. (2002) 66:506-577).
US20090053777 discloses a process for saccharification of pretreated biomass to obtain high concentrations of fermentable sugars. The fed batch reactor system includes multiple size reduction steps and mixing to maintain through mixing in a vertical, agitated tank. The process comprises of: providing a portion of mixable pretreated biomass slurry; & portion of saccharification enzyme consortium comprising at least one enzyme capable of hydrolyzing cellulose; reacting said slurry and enzyme at temperature ranging from 25° C. to 60° C. and pH 4.5 to 6.0; applying the article size reduction mechanism; adding an additional portion of pretreated biomass producing a higher solid biomass slurry; reacting said higher solid biomass slurry under same mentioned conditions, wherein repeating the step two more times to produce a high sugar content hydrosylate; where the dry weight of pretreated biomass is in between 24% to 30% to obtain 20% of the weight of the final hydrosylate product.
WO2011157427 describes a continuous process for enzymatic hydrolysis of cellulosic biomass, wherein the process involves addition of predetermined amount of cellulosic biomass and enzyme in a continuously stirred tank reactor to partial enzymatic hydrolysis of cellulosic biomass, wherein the partially hydrolysed cellulosic biomass is continuously removed. The said cellulosic biomass has a solid content in between 10 to 45%.
WO2006063467 discloses a continuous process system for enzymatic hydrolysis of pre-treated cellulose which comprises: introducing aqueous slurry of the pre-treated cellulosic feedstock at the bottom of a vertical column hydrolysis reactor. Axial dispersion in the reactor is limited by avoiding mixing and maintaining an average slurry flow velocity of about 0.1 to about 20 feet per hour, such that the undissolved solids flow upward at a rate slower than that of the liquid. Cellulase enzymes are added to the aqueous slurry before or during the step of introducing the aqueous slurry into the reactor. An aqueous stream comprising hydrolysis products and unhydrolyzed solid is separated and unhydrolyzed cellulose is recycled in same reactor. It also describes the enzyme compositions comprising of cellulase enzymes and flocculents to provide exposure of the enzyme to the substrate for hydrolysis process. The time required for cellulose conversion to glucose is 48 to 200 hrs at respective enzyme loading of 32 units/g cellulose to 5 units/g cellulose.
US20100255554 discloses a method for optimization of a fed batch hydrolysis process wherein the hydrolysis time is minimized by controlling the feed addition of volume and/or batch addition frequency or prehydrosylate and enzyme feed. The mentioned process comprises: filling a reactor vessel with water; adding cellulase enzyme; and sequentially adding lignocellulosic prehydrosylate biomass feed into reactor vessel to produce a reaction mixture, whereby prehydrosylate feed is added in batches at a preselected batch volume and batch addition frequency over a total feed time of 20 hrs to achieve a preselected final consistency and preselected dry matter content in a final reaction mixture. 70 to 90% of a theoretical cellulose to glucose conversion is reached in the reaction mixture, wherein batch addition frequency is one batch every 80 to 105 mins, preselected final consistency is 24%, total feed time is 80 to 120 hrs.
According to the methods described in the prior art the major strategy used in the biofuels production includes three main steps i.e. biomass treatment, enzymatic hydrolysis, and fermentation of sugars to produce biofuels. The main obstacles faced during enzymatic hydrolysis are low rate of reaction, high cost of enzyme, low product concentration. As per the methods described in the prior art, the above described problems is overcome by operating the enzymatic hydrolysis using high insoluble solid consistency. However, the saccharification reaction at high insoluble solid consistency will have to encounter challenges of increased viscosity, higher energy requirement for mixing, shear activation of enzyme and poor heat transfer due to rheological properties of dense fibrous suspension. Thus there is a need to develop a process which can overcome the above challenges.